1. Field of the Invention
The present invention generally relates to phase shift masks, and particularly, to a structure of a light-attenuating phase shift mask attenuating exposure light and a method of manufacturing the same.
2. Description of the Background Art
Recently, high integration and miniaturization have been greatly developed in semiconductor integrated circuits. Accordingly, miniaturization of circuit patterns formed on a semiconductor substrate (hereinafter simply referred to as a "wafer" has been developed rapidly. A photolithography technique is, among others, well known in the art as a basic technique for pattern formation, for which various developments and improvements have been made. However, there is still an increasing need for miniaturization of a pattern, and thus, there is still a strong need for improvement in resolution of a pattern.
In general, a resolution limit R(nm) in the photolithography technique using a demagnification exposure method is expressed as EQU R=k.sub.1 .multidot..lambda./(NA) (1)
where .lambda. represents a wavelength (nm) of light to be used, NA represents a numerical aperture of a lens, and k.sub.1 is a constant depending on a resist process.
As can be seen from the above expression, a resolution limit is improved as far as values of k.sub.1 and .lambda. are made smaller and a value of NA is made larger. In other words, it is sufficient to reduce the constant depending on the resist process with the wavelength being shorter and NA being increased. However, it is difficult technically to improve a light source or a lens, and resolution is degraded because a depth of focus 6 of light (.delta.=k.sub.2 .multidot..lambda./(NA).sup.2) is made smaller by shortening the wavelength and increasing NA.
With reference to FIG. 18, description will now be made of a cross section of a photomask, electric field on the mask, and a light intensity on a wafer, when a conventional photomask is used.
Referring to FIG. 18(a), a structure of a photomask 30 will be first described. A mask pattern 38 having a predetermined shape is formed on a transparent glass substrate 32. Mask pattern 38 includes a light shielding portion 34 formed of chromium or the like and a light transmitting portion 36 from which transparent glass substrate 32 is exposed.
Referring to FIG. 18(b), the electric field of exposure light on photomask 30 is provided along the photomask pattern.
Referring to FIG. 18(c), the light intensity on a semiconductor wafer will be described. When a fine pattern is to be transferred, beams of exposure light transmitted through the photomask intensify with each other in a portion of adjacent pattern images where beams of light are overlapped, because of diffraction and interference.
Therefore, a difference in the light intensity on the semiconductor wafer is reduced, so that a resolution is deteriorated. As a result, a pattern transferred onto a resist film cannot reflect the photomask pattern precisely as shown in FIG. 18(d).
In order to solve this problem, a phase shift exposure method using a phase shift mask is proposed in Japanese Patent Laying-Open Nos. 57-62052 and 58-173744.
With reference to FIG. 19, a phase shift exposure method using a phase shift mask disclosed in Japanese Patent Laying-Open No. 58-173744 will now be described.
Referring to FIG. 19(a), a structure of the phase shift mask will be described. A phase shifter 40 formed of a transparent insulating film such as a silicon oxide film is provided at every other light transmitting portion 36 of a mask pattern 38 formed on a glass substrate 32.
Referring to FIG. 19(b), the electric field on the phase shift mask formed by beams of light transmitted through phase shifter 40 has phases converted alternately by 180.degree.. Therefore, in adjacent pattern images, overlapping beams of exposure light transmitted through phase shifter 40 have phases converted from each other. Accordingly, the amplitude of light on a resist mask is as shown in FIG. 19(c). As to the light intensity on the resist film, beams of light are canceled with each other due to interference in a portion where beams of light are overlapped, as shown in FIG. 19(d). As a result, there is provided a sufficient difference in the light intensity on the resist film, allowing improvement of the resolution, so that a pattern reflecting the mask pattern can be transferred onto the resist film as shown in FIG. 19(e).
However, although the above-described phase shift mask is highly effective with respect to a periodic pattern such as line and space, arrangement of phase shifters and the like becomes very difficult in the case of a complex pattern, so that the phase shifter cannot be arbitrarily set.
The inventors of the present invention disclose a light-attenuating phase shift mask in Japanese Patent Application No. 5-91445. The light-attenuating phase shift mask disclosed in Japanese Patent Application No. 5-91445 will be described with reference to FIG. 20.
A light-attenuating phase shift mask 200 includes a quartz substrate 50 transmitting exposure light, and a phase shift pattern 60 formed on the main surface of quartz substrate 50. Phase shift pattern 60 includes a light transmitting portion 51 from which quartz substrate 50 is exposed, and a phase shift portion 52 formed of a single material converting the phase angle of the exposure light by approximately 180.degree. and having a transmittance of 3-20% with respect to the exposure light transmitting through light transmitting portion 51.
Referring to FIG. 21, description will now be given of electric field on a mask of exposure light transmitted through phase shift mask 200 as structured above, the amplitude of light on a resist film, the light intensity on the resist film, and a pattern to be transferred onto the resist film.
FIG. 21(a) is a sectional view of phase shift mask 200. The electric field on the mask has a phase converted at an edge portion of the exposure pattern as shown in FIG. 21(b), thus providing the amplitude of exposure light on the resist film as shown in FIG. 21(c). Therefore, the light intensity on the resist film is necessarily 0 at the edge portion of the exposure pattern, as shown in FIG. 21(d). As a result, there is provided a sufficient difference in the electric field of exposure pattern between light transmitting portion 51 and phase shifter portion 52 so as to obtain high resolution, whereby the pattern reflecting the phase shift pattern can be transferred onto the resist film as shown in FIG. 21(e).
Description will now be given of a method of manufacturing phase shift mask 200 using a molybdenum silicide film or a molybdenum silicide nitride oxide film as a phase shifter film.
FIGS. 22 to 25 are sectional views showing the manufacturing process of phase shift mask 200 shown in FIG. 20.
Referring to FIG. 22, a phase shifter film 52 of a molybdenum silicide oxide film or a molybdenum silicide nitride oxide film is first formed on quartz substrate 50 with a sputtering method. Then, in order to stabilize the transmittance of phase shifter film 52, heating at a temperature of 200.degree. C. or more is carried out using a clean oven or the like. Because of this heating, a transmittance variation (0.5-1.0%) caused by heating (at approximately 180.degree. C.) such as a resist application step at formation of phase shifter film 52 can be prevented in advance.
In order to prevent charging-up at the time of exposure of an electron beam resist film, to be formed later, by electron beams, a charge dissipating film 61 formed of molybdenum or the like of approximately 100 .ANG. thick is formed. Then, an electron beam resist film 53 (ZEP-810S.RTM. manufactured by Nippon Zeon Co., Ltd.) of approximately 5000 .ANG. thick is formed on charge-dissipating film 61.
Referring to FIG. 23, electron beams are directed at a predetermined position of electron beam resist film 53. By developing resist film 53, resist film 53 having a predetermined resist pattern is formed.
Referring to FIG. 24, charge-dissipating film 61 and phase shifter film 52 are etched with resist film 53 used as a mask. This etching is carried out with a parallel plate type RF ion etching device, with the distance between electrode substrates set to 160 mm, the working pressure set to 0.3 Torr, the reactive gas of CF.sub.4 and O.sub.2 having flow rates of approximately 95 sccm and approximately 5 sccm, respectively, and the etching time set to approximately 12 minutes.
Then, referring to FIG. 25, resist film 53 and charge dissipating film 61 are removed. Accordingly, phase shift mask 200 disclosed by Japanese Patent Application No. 5-91445 is completed.
However, the above prior art has the following problems.
Referring to FIG. 26, phase shifter film 52 formed of a molybdenum silicide oxide film (solid line A in the figure) or a molybdenum silicide nitride oxide film (solid line B in the figure) has a transmittance of 10% or less with respect to a krF laser (248 nm), while it has a transmittance of 30% to 60% with respect to light having a wavelength of 488 nm of a defect inspection apparatus used for inspection of a defect of a phase shift mask, for example.
Therefore, when an ordinary defect inspection apparatus (KLA 3 series) using light having a wavelength of 488 nm is used for inspection of a phase shift mask, detection sensitivity for a defect generated on a phase shift mask becomes small.